The present invention relates to a new and improved construction of a target tracking system.
In its more particular aspects the present invention specifically relates to a new and improved construction of a target tracking system containing at least one angle sensor for measuring the angular deviation of the target from at least one line of sight associated with at least one antenna of the at least one angle sensor. There are further provided at least one range sensor for measuring the range between the target and the at least one range sensor and a servo system for continuously laying the at least one antenna of the at least one angle sensor and the at least one range sensor towards the target such that the target is continually present within the transmitting range and the receiving range of the at least one angle sensor and the at least one range sensor.
Target tracking systems of the aforementioned type are primarily required for temporally and spatially measuring and extrapolating the flight paths or trajectories of civil or military flying objects, such as aircraft and missiles within the atmosphere. The tracked or measured objects, however, may also move in outer space, on the ground or on water and even under water.
Regarding non-military uses it is intended in most cases to prevent a collision of two objects, for example, commercial air planes. Regarding military applications, however, it is intended in most cases to bring about the collision of two objects like, for example, a projectile and a target.
For this purpose, the objects must be tracked and measured in the most precise and most reliable manner as possible because the extrapolation of a flight path or trajectory cannot be better than the tracking and measurement of such flight path for trajectory.
It is for this reason that frequently a plural number of sensors is utilized and such plural number of sensors define a common or at least overlapping measuring range for tracking and measuring a single object or target. It will be assumed for the description following hereinbelow that only one object or target is intended to be tracked by each target tracking system.
The operation of tracking implies that the sensors or their antennae are moved by means of a servo system such that the object or target to the greatest possible extent is continually present within the measuring range of the aforementioned sensors or their antenna. Only in this manner is it possible for the object or target and its flight path or trajectory to be measured sufficiently effectively and only in this manner is it possible for the object or target, in turn, to be tracked sufficiently effectively. Thus the tracking operation and the measuring operation are mutually dependent with respect to the quality of their results.
In most cases, the sensors transmit and receive electromagnetic waves having a wavelength in the range between 10.sup.-7 m and 10.sup.-1 m. However, acoustic waves are also utilized, particularly under water or in submarine applications. Acoustic waves propagate in air at a velocity which is smaller than the propagation velocity of electromagnetic waves by a factor of about 1 million or 10.sup.6.
The measuring ranges of the sensors are relatively narrow in most cases and define a center line which will be called "line of sight" further hereinbelow. Such lines of sight must not absolutely be straight. For example, the lines of sight can be curved when acoustic waves traverse layers of air having different air temperatures and/or different wind velocities.
The aforementioned range sensors generally measure the range or distance between such range sensor and an object or target on the basis of the travelling time of the employed waves from the range sensor to the object or target and back to the range sensor. For this reason, the range sensors must constitute active range sensors, i.e. sensors which themselves transmit the waves. The lines of sight associated with the waves transmitted by the transmitter and received by the receiver of such range sensor must be permitted to be moved independent of each other in the event that the object or target does not move much slower than the transmitted or received wave.
Contrary thereto, angle sensors measure the angular deviation of the object or target from the line of sight of the angle sensors which also may constitute passive sensors, i.e. receive waves which originate from external sources, for example, the sun or the object or target itself. The ratio of the wavelength of the waves and the size of the antenna receiving and possibly also transmitting the waves, greatly affects the narrowness of the measuring range or the laying action of the angular sensors and thus particularly the precision of such angle sensors. This laying action is also required in order to maintain small the energy requirements when active angle sensors are utilized. This implies specifically at greater wavelengths that relatively heavy antennas and their lines of sight must precisely track the object or target. The imprecision of such tracking operation is frequently also called "tracking error" and impairs the precision of the angle sensors which, in turn, increases the tracking error and so forth.
Thus, in order to precisely measure the object or target, and specifically the reflection center of the object or target and clearly differentiate or discriminate between adjacent objects or targets or the object or target and its mirror image, it is important that the target tracking signals produced by the sensors are processed and supplied to the servo system in a manner such that the servo system has the lines of sight of the sensors pass through the object or target as reliably and precisely as possible. It is particularly intended to maximize the probability that the object or target is continually located within the measuring range of the sensors. This implies that the precedingly obtained measuring values of the sensors control the servo system such that no future measuring value is lost due to tracking errors. During such operation there can also be accounted for and minimized the load on the servo system due to, for example, wear, heating and the like.